Acute renal replacement therapy is a treatment for patients suffering from lost or reduced renal function, while apheresis therapy is a treatment for exchanging the plasma part of the blood. During the therapy, blood is led from the patient into an extracorporeal blood circuit, and metabolic waste products including uremic toxins and fluids, or plasma are removed from the blood. This involves the transfer of substances with convection and/or diffusion across a semipermeable membrane provided in a hemodialyzer or hemofilter or plasmafilter. The cleaned blood is then returned from the extracorporeal blood circuit to the patient.
In some blood purification therapies a solution fluid (dialysate) which comprises an aqueous solution of glucose and electrolytes is introduced from a solution circuit to the non-blood side of the semipermeable membrane.
Furthermore in some blood purification therapies any fluid lost during the therapy is partially or wholly replaced by the introduction of a solution fluid from the solution circuit directly into the blood in the extracorporeal blood circuit. In the cases of hemofiltration and plasma exchange therapy, the solution is substitution solution and plasma replacement solution respectively.
Hemodialysis, hemofiltration, and hemodiafiltration are renal replacement therapies which apply a hemodialyzer or hemofilter, while plasma exchange therapy applies a plasmafilter
The dialysate, substitution solution or plasma replacement solution is hereinafter called solution. The hemodialyzer or hemofilter or plasmafilter is hereinafter called filter.
During a blood purification therapy one important task is to maintain the solution at a desired temperature before infusing it via the filter and/or introducing it directly into the blood, as the case may be. In clinical applications the solution is commonly provided in sterile bags which are stored at room temperature in a stock room of the clinic and are taken out when needed. The solution is then warmed to body temperature so as not to cool the patient's blood when the solution communicates with the blood either via the membrane of the filter, or directly via the extracorporeal blood circuit. Cooling of the blood in the extracorporeal blood circuit can lead to discomfort and, in the worst case, cardiac arrhythmia, which can occur especially when blood temperature falls below 33° C. It is known to warm the solution inline as it passes through the blood purification therapy equipment.
In order to maintain sterility the solution is conducted through a sterile disposable conduit or conduits, and there are known teachings for heating the solution with a warmer, wherein a heating element is provided to make contact on a surface of the disposable conduit. Thus the solution is heated inline, that is to say it is heated as it passes through the solution circuit, with thermal conduction. Temperature control is provided by temperature sensors placed preferably, for reasons of cost, externally to the disposable conduit, rather than directly in contact with the solution.
In the case of these known warmers the heating element has a high thermal inertia, in particular since it tends to have a large heating surface. Also the temperature of the heating element surface must be higher than the desired solution temperature in order to ensure efficient heat flow. In the case that the solution delivery is stopped by the user or by an alarm signal, for example, there is the possibility of temporary and local overheating of the solution even when power to the warmer is stopped. At this time the respective temperatures of the solution and heating element are equalizing. Therefore a situation can occur when the solution temperature is uncontrolled. Various operating factors influence the possibility and degree of overheating of the solution, such as surface temperature of the heating element, ambient air temperature, temperature of the sterile bag, and desired solution temperature. Overheating of the solution can lead to thermal breakdown of the solution and/or thermal hemolysis of the patient's blood.